Extruded foam molded by extrusion-foaming a thermoplastic resin and an assembly of bundled threads of the extruded foam molded by a so-called strand-extrusion involving the steps of extruding the thermoplastic resin from dies having a large number of small pores; bundling extruded resin threads together; and fusing and foaming the surfaces thereof are excellent in mechanical properties even though light in weight. Therefore, the foam is broadly applied as structural materials in various fields, such as the fields of building construction, civil engineering and the fields of automobiles, in particular, employed as thermal insulating materials. As such extruded foam of a thermoplastic resin extruded foam formed of polyurethane-based resins or polystyrene-based resins is known.
However, a polyurethane resin and a polystyrene resin are materials that are not always excellent in recycling characteristics, and there is a problem that when these resins are used, it is difficult to sufficiently comply with the construction recycling law (law on recycling of materials for construction works, etc.). In addition, the polystyrene resin has poor heat resistance and chemical resistance. Therefore extruded foam made of a thermoplastic resin that is alternative to those resins has been demanded.
On the other hand, a polypropylene-based resin, which is excellent in mechanical property, heat-resisting property chemical resistance, electrical property and the like, is also a low cost material, so that it is widely used in various molding fields. Thus, extruded foam of the polypropylene-based resin is also expected to have high industrial utility. However when melted, polypropylene, which is a linear polymer resin, may decrease the strength as a result of a drastic decrease in viscosity, and consequently the resin hardly holds the air bubbles of foam and foam braking can easily occur. Therefore, it is difficult to obtain extruded foam having as high a closed cell content air bubbles and high an expansion ratio as those of the conventional thermoplastic resins. In addition, it is difficult to uniform and densify the diameters of the foam cells (air bubbles) of the extruded molded product. Therefore, an improvement in moldability has been desired.
Here, insulation efficiency in a case that the extruded foam is used as a thermal insulation material depends on both the expansion ratio and the cell diameter at a certain range of expansion ratio (for example, 10 or more). In other words, a higher expansion ratio leads to an enhanced insulation efficiency because the thinner a material wall is, the smaller a heat-transfer amount is. Similarly, when the cell diameter is reduced with the same expansion ratio, the number of cell walls blocking radiant heat increase, so that less heat is conducted, thereby improving heat insulation properties. Accordingly, the smaller cell diameter is preferred. As described above, in a state where expansion ratio is made high, when an average cell diameter is made small to improve the heat insulation efficiency a thickness of molded product can be reduced, which also brings an additional effect of cost reduction. Therefore, notwithstanding the difficulty of the moldability as described above, there has been a need of an improved expansion ratio and a decreased cell diameter.
Under such a circumstance, researches have been carried out to improve the expansion ratio and reduce the cell diameter of the extruded propylene-based resin foam. For example, there is provided an assembly of bundled threads of extruded foam made of a polypropylene-based resin (i.e., a structural material of the resin foam), the structural material having a biaxial extensional viscosity of 3×106 poises or more and a biaxial strain hardening rate of 0.25 or more at a biaxial extensional strain of 0.2 and (see, Patent Document 1). In addition, there is also provided an assembly of bundled threads of an extruded foam obtained through steps of melt-kneading a mixture of a predetermined polypropylene-based resin and a foaming agent in an extruder; extrusion-foaming the mixture from the extrusion die having a plurality of small pores to be formed into threads under low-temperature and low-pressure conditions after the temperature is adjusted suitably for foaming; and transferring the threads into a cylindrical device for bundling and fusing while the threads of the foam is softened (see, for example, Patent Document 2).
[Patent Document 1] JP 09-25354 A
[Patent Document 2] JP 2001-1384 A